The invention relates to a method for determining the ability of a vehicle to start, in particular during the starting process of the vehicle.
In vehicles, safety-related functions are being increasingly operated electrically, for example electric brakes. In order to ensure the operational capability of such safety-related components, they must be continuously supplied with electrical energy. To do this, it is known to provide two energy sources, specifically a battery and a generator, in the vehicle.
When the vehicle is turned off, the battery is usually loaded. Discharging of the battery may give rise to such a low battery charge state that starting or other functions of the vehicle are no longer ensured. Reliable information on the battery charge state is therefore necessary.
European Patent EP 0 916 959 A2 (U.S. equivalent, U.S. Pat. No. 6,118,252) discloses a method for determining the ability of the starter battery of a vehicle to start, in which method the voltage dip when the vehicle starts is compared with voltage values of a characteristic curve diagram. A display or alarm function is triggered when there is a deviation. Furthermore, German Patent DE 37 12 629 C2 discloses a measuring device for the remaining service life of a vehicle battery in which, by reference to the terminal voltage and the load current of the battery, the internal resistance of the battery is determined in a temperature-compensated fashion before and after the vehicle is started. By forming a ratio and by reference to a comparison with a limiting value, a signal is generated which indicates the battery service life which has been determined.
The object of the present invention is to specify a method for determining the ability of a vehicle to start which permits the operational capability of the battery to be detected in a particularly simple and reliable way. Furthermore, a particularly simple device for determining the ability to start is specified.
The object is achieved according to the invention by means of a method for determining the ability of a vehicle to start with current values and voltage values of a battery being sensed during a predefined time window, in particular while the vehicle is being started. An internal resistance of the battery is determined by reference to the current values and the voltage values by means of linear regression. A reference resistance value for the starting behavior of the vehicle is determined by reference to a maximum value for the current and a minimum value for the voltage which are sensed within the predefined time window. A value which represents the ability to start is determined by reference to a comparison between the internal resistance and the reference resistance value. The internal resistance of the battery is preferably determined by reference to the current and voltage values which are measured during the starting behavior. For this purpose, in particular the current values which represent the falling edge and the voltage values which represent the rising edge are determined. These values are used to determine the internal resistance by means of linear regression. The reference resistance value is preferably determined within a time window of the starting phase by reference to the maximum value for the current and the minimum value for the voltage which represent this time window. This ratio represents the resistance of the starter circuit, i.e. of the engine. Comparing the reference resistance value of the starter circuit with the internal resistance value of the battery permits reliable conclusions concerning the ability of the battery to start, without a complex measuring operation. In particular, the method for directly assessing the ability of the vehicle to start is made possible by reference to the operational capability of the battery. Furthermore, the method can be used independently of the type of vehicle or size of battery.
The value for the internal resistance of the battery is expediently monitored to determine whether it is equal to or drops below the reference resistance value of the starter circuit. When the value of the internal resistance is equal to or drops below the reference resistance value by at least 1% the engine is able to start.
If the value for the internal resistance of the battery is equal to or greater than the reference resistance value, a signal message is advantageously output. This ensures that, before the ability of the battery to start is lost, a signal message which indicates the critical state of the battery is output so that measures which are necessary for the ability to start can be timely mode.
The temperature of the battery and/or a starter circuit of the vehicle is preferably determined. This permits both the reference resistance of the starter circuit and the internal resistance of the battery to be determined as precisely as possible based on their dependence on temperature. As an alternative or in addition it is possible to determine the ability of the battery to start at a different temperature, and thus make predictions, on the basis of a currently sensed temperature of the battery and a currently sensed temperature of the starter circuit, in particular of the engine.
A value determined as a function of an instantaneously determined temperature of the battery and/or of the starter circuit, or a value which has been determined at a given time, is used to determine, in particular interpolate and extrapolate, the value for the internal resistance or reference resistance value which represents the ability to start at a different temperature, and thus at a later time. As a result, the ability of the battery to start can be predicted by taking into account the temperature profile over time. This ensures that the ability to start at a later time can be assessed when the temperature is dropping, in particular when the temperature of the starter circuit is dropping, for example at the temperature of the engine at which the starting power rises disproportionately.
The charge state of the battery is advantageously monitored and linked to the internal resistance of the battery in such a way that when its value drops below a limiting value for the internal resistance in a given charge state, the battery is disconnected from the on-board electrical system. This ensures that the battery is disconnected from the on-board electrical system in sufficient time when said battery is in a critical state. In particular, in the case of a relatively long inactive time of the vehicle during which the battery is permanently discharged, a critical state is detected in good time so that the battery can be disconnected if appropriate on-board electrical system.
The second-mentioned object is achieved according to the invention by means of a device for determining the ability of a vehicle to start with a current sensor for sensing current values and a voltage sensor for sensing voltage values of a battery during a predefined time window, in particular while the vehicle is being started. An evaluation unit is provided for determining an internal resistance of the battery by reference to the current values and the voltage values by means of linear regression. The evaluation unit also determines a reference resistance value for the starting behavior of the vehicle by reference to a maximum value for the current and a minimum value for the voltage which are sensed within the predefined time window. It is thus possible to determine a value which represents the ability to start by reference to a comparison between the internal resistance and the reference resistance value. This provides a device which is of particularly simple design and which can be used independently of the type of vehicle or type of battery.